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Influence of Moisture on Rice Straw Decomposition in Soils¹

ABSTRACT

Laboratory incubation experiments were conducted for 4 months at 22 ± 0.5C with two fine-textured soils amended with doubly tagged (¹³C and ¹⁵N) rice straw (Oryza sativa L.) under different moisture regimes. The moisture levels were 30, 60, and 150% of water-holding capacity, (WHC). The extent of straw decomposition was found to be a power function of time. Differential equations were derived to give rate functions for each moisture level.

Native soil organic carbon loss was depressed by added energy materials when either moisture or oxygen supply limited decomposition. At 30% WHC, priming ratios were < 1.0 initially, and increased with time of incubation but did not reach unity within 4 months incubation. At 60% WHC, the ratios were > 1.0 and decreased with time of incubation but remained above 1.0 throughout incubation. At 150% WHC, priming ratios were < 1.0 and did not vary much during the entire decomposition period. The net release of straw-N at 60% WHC was higher than at 30% WHC. At 150% WHC, straw-N released during decomposition was lost from the system, presumably by denitrification. Soil inorganic nitrogen changes followed trends similar to net release of straw-N. The influence of moisture variables on soil organic nitrogen transformations was more pronounced than the influence of decomposing straw.

¹ Contribution from the Dep. of Soils and Plant Nutrition, Univ. of Calif., Davis, 95616. This work was supported in part by a grant from the California Rice Research Board.

² Postgraduate Research Soil Scientist and Professor of Soil Microbiology, respectively.

Received for publication July 22, 1974. Accepted for publication October 7, 1974.